Don't send machine and user ID to Omaha anymore. Send a/r pings instead.

This avoids sending a unique ID in order to track active user counts.
Note that this CL doesn't remove the machine/user/Omaha ID/file from
the params object -- it just makes them unused/obsolete. Removal will
be done in a subsequent CL in an effort to make this CL smaller.

BUG=1439
TEST=unit tests, x86-generic, arm-generic, gmerged and inspected logs

Review URL: http://codereview.chromium.org/2856070
diff --git a/main.cc b/main.cc
index 204c525..bdd216c 100644
--- a/main.cc
+++ b/main.cc
@@ -14,6 +14,7 @@
 #include "metrics/metrics_library.h"
 #include "update_engine/dbus_constants.h"
 #include "update_engine/dbus_service.h"
+#include "update_engine/prefs.h"
 #include "update_engine/update_attempter.h"
 
 extern "C" {
@@ -105,11 +106,16 @@
   // Create the single GMainLoop
   GMainLoop* loop = g_main_loop_new(g_main_context_default(), FALSE);
 
+  chromeos_update_engine::Prefs prefs;
+  LOG_IF(ERROR, !prefs.Init(FilePath("/var/lib/update_engine/prefs")))
+      << "Failed to initialize preferences.";
+
   MetricsLibrary metrics_lib;
   metrics_lib.Init();
 
   // Create the update attempter:
-  chromeos_update_engine::UpdateAttempter update_attempter(&metrics_lib);
+  chromeos_update_engine::UpdateAttempter update_attempter(&prefs,
+                                                           &metrics_lib);
 
   // Create the dbus service object:
   dbus_g_object_type_install_info(UPDATE_ENGINE_TYPE_SERVICE,